1. Field of the Invention
The present invention relates to a time division multiple access (TDMA) system receiver for intermittently receiving burst data.
2. Description of the Related Art
A communication system called a time division multiple access (TDMA) communication system is applied to a digital radio telephone system. FIG. 1 is a diagram showing an example of a frame arrangement according to the TDMA communication system. In FIG. 1, one frame is divided into a plurality of time slots (eight time slots in this arrangement). Of the eight time slots, four time slots are allocated to transmission slots T1, T2, T3, and T4, and the other four time slots are allocated to reception slots R1, R2, R3, and R4. In the TDMA communication system, data is transmitted and received between two transceivers by employing the slots T1 and R1 as the transmission and reception slots respectively, and at the same time data is transmitted and received by two other transceivers by also respectively employing the slots T2 and R2 as the transmission and reception slots. As described above, when one frame is composed of eight time slots, it is possible to transmit and receive data of four transmission-reception systems at maximum capacity with one channel.
Other than the above communication system in which the data is transmitted and received with the same channel (frequency), there is a communication system in which a transmission channel and a reception channel are independently provided and a time division multiple access communication is carried out in each channel.
When such a communication system is employed, an offset amount of a carrier frequency of a received signal is detected by comparing phases of channel responses between two points in a burst data received in each time slot.
FIG. 2A shows an example of the burst data transmitted in each time slot. In FIG. 2A, the burst data includes a tail bit portion, a first half of a data bit portion, a sync. bit portion, a second half of the data bit portion, and a tail bit portion which are arranged therein in that order. The sync. bit portion is located at the center of one burst. The tail bit portion and the sync. bit portion are data having predetermined patterns.
Accordingly, it is easy to detect the tail bit portion and the sync. bit portion from the received data. It is also possible to detect an offset amount of the carrier frequency based on the detected phases of the tail bit portion and the sync. bit portion. Specifically, as shown in FIG. 2B, there is detected a position ta at which a value of a correlation function between the head tail bit portion of the received data and the data of the tail bit portion previously stored in a receiver become maximum.
There is also detected a position tb at which a value of a correlation function between the sync. bit portion of the received data and data of the sync. bit portion previously stored in the receiver becomes maximum.
The difference between the phases of the two detected positions ta and tb (phase difference relative to a phase of the carrier frequency) is detected to calculate the offset amount of the carrier frequency. The received signal is corrected by the offset amount of the carrier frequency to obtain a precise received data.
However, when the offset amount of the carrier frequency is detected and processed as described above, the detected offset amount of the carrier frequency is not sufficiently large as compared with an offset amount caused by other factors such as a noise or the like.
Therefore, accuracy with which the offset amount of the carrier frequency is detected is low.
In other words, since in the above TDMA communication system the burst data is received intermittently, time difference between the two positions in one burst data is not so large (e.g., one burst is about several 100s of .mu. second). Therefore, it is difficult to satisfactorily detect the offset amount of the carrier frequency without any influence of noise.
The one burst includes known bit patterns of the tail bit portion and the sync. bit portion as described above. Since the tail bit portion has a small number of bits, it is difficult to detect a sufficient correlation from the tail bit portion. When the tale bit portion is used to detect the phase difference, it is difficult to obtain a sufficient detection accuracy. In order to avoid such problem, it is possible to consider that the phase difference between two positions in the sync. bit portion of the one burst is detected. However, in this case, since the time between the two positions is very short, the influence of the noise upon the detected phase difference is more pronounced.